This invention relates to a composition and process for enhancing silver images, and, more particularly, for enhancing photographically produced silver images for use in lithography.
In the silver salt diffusion transfer process, an imagewise exposed silver halide emulsion layer is positioned in intimate contact with a receptor element and developed with a silver halide developing composition in the presence of a solvent for silver halide such that the unexposed silver halide is dissolved and subsequently diffused into the receptor element. Development nuclei, or catalyst, contained in the receptor element, cause the diffusing silver halide to be reduced (i.e., developed) to metallic silver, thereby forming an image on the surface of the receptor element.
As applied to the photolithographic process, a developing composition containing a developing agent, e.g., hydroquinone, a solvent for silver halide, e.g., the thiosulfate ion, and, preferably, an antifoggant, e.g., benzotriazole, is used. When the imagewise exposed silver halide emulsion layer is contacted with the developing composition, the exposed silver halide grains are reduced to silver metal in normal fashion. They remain immobilized in the emulsion layer. The unexposed silver halide grains dissolve in the developing composition via formation of soluble silver complexes, such as the complexes of silver thiosulfate. When the soluble silver complex contacts a development nuclei or catalyst in the receptor element, the silver ion is reduced to a metallic deposit which can then form the ink receptive image areas of a lithographic plate.
There are many variations on this theme, such as: some or all of the developer may be initially incorporated within the plate structure; the plate may contain either a positive or negative silver halide emulsion; the development nuclei or catalyst-containing layer may be on a separate support or receptor element and brought into intimate contact with the silver halide emulsion after being wet with developer composition, following which the two separate supports are peeled apart; a receiving layer containing development nuclei or catalyst can be contained as an intermediate layer between the initial support and the silver halide emulsion, such as is taught in U.S. Pat. No. 4,204,868; or the receptor layer can be contained on top of the silver halide emulsion layer, as is taught in U.S. Pat. Nos. 3,728,114 and 4,160,670.
If the silver halide emulsion layer is placed between the support and the catalyst layer, then the portions of the plate where the silver image is not bared are composed of protective colloids such as gelatin, polyvinyl alcohol, etc. and are hydrophilic while the portions where the silver image is bared are oleophilic due to the deposited silver and thus have an affinity to ink. If the catalyst layer is placed between the support and the silver halide emulsion layer, then the emulsion layer is subsequently removed to reveal a positive silver image on the catalyst layer. A hydrophilic support is used to provide the required lithographic differential.
One shortcoming of the silver diffusion transfer process is that a substantial level of silver halide is required to provide a positive silver image of sufficient oleophilicity to function as a lithographic plate. Another shortcoming is that there is often insufficient oleophilic/hydrophilic differential between the image areas and background areas of the lithographic plate.
There have been several attempts in the prior art to increase oleophilicity and thus improve lithographic functionality of plates produced by the silver diffusion transfer process. One means of increasing oleophilicity, silver amplification, is typically provided by a "redox" solution in which an oxidizing agent and a reducing agent are placed together. These agents are chosen so that the rate of spontaneous reaction between them will be slow except in the presence of a catalyst (silver) which increases the rate of spontaneous reaction dramatically. Consequently, reaction will tend to occur mainly in areas where catalyst is present, and if one or more of the redox reaction products becomes insolubilized, a build-up of oleophilic solid material will occur in those areas.
However, all catalytically activated redox processes suffer from the same problem--since the reaction ingredients (oxidizing agent and reducing agent) are thermodynamically unstable toward one another in the presence of a catalyst, contamination by the catalyst will cause rapid degradation of the processing solution. In addition, most such solutions contain oxidizing agents and reducing agents which are thermodynamically unstable toward one another even in the absence of a catalyst. Many processing solutions suffer from still another problem in that the products of spontaneous reaction between the oxidizing and reducing agents act as catalysts which further accelerate the spontaneous reaction. This is known as autocatalytic degradation.